Floodplain wetlands provide habitat for a diverse range of aquatic biota, as well as
performing other important ecosystem functions such as transformation of nutrients, providing breeding and nursery grounds for numerous species. Overall productivity and biodiversity of floodplain wetlands are closely linked with water availability, and in particular to the reliable reoccurrence of water inflows. Alteration of wetland inflow and outflow regimes can greatly impact the functioning of food-webs through biodiversity loss, diversion of energy flow and ecosystem functionality. In the present study stable isotope ratios (δ13C and δ15N) of primary producers, sediment organic matter (SOM) and a variety of invertebrate and fish species are used to gain better understanding of the food-web relations at various waterholes in
wetter and drier conditions in Yanga National Park, in the lower Murrumbidgee
floodplain. Mass-balance mixing models were used to examine relative food source
contributions to consumer diet. We also employed probabilistic simulation software to
better understand trends of trophic positions, diet shifts and varying contributions from sources to consumers in waterholes of the Yanga wetlands. We compared δ13C and δ15N data from wetter (greater surface water area and depth)
conditions in February, with data from the drier (smaller surface water area and
depth) conditions in August. The data indicated contraction in the trophic position of Hypseleotris spp. (carp guudgeon) in August consistent with shrinking waterholes where species were forced into competing for overlapping, and a more limited variety, of food sources. In particular the endemic Hypseleotris spp. appears to be forced into greater competition with the exotic C. carpio (carp).
Data also indicated that energy source of consumer species varied with changing
water levels among all waterholes. The drier (August) δ13C values for fish and
insects were typically shifted ~1-3‰ lower than the corresponding wetter (February)
values. These shifts appear to correspond to depletion in the δ13C algal values, and
appear to indicate a greater proportional contribution of δ13C from algae to the δ13C
in consumers. This study provides δ13C and δ15N values for endemic and exotic aquatic species in Australian semi-arid wetlands. It provides modelling results indicating shifts in energy source and trophic position relative to water fluctuations and indicates increased competition among similar species that may adversely impact endemic species
populations.